1. Field
This invention relates generally to die temperature sensor circuits and more specifically to a die temperature sensor circuit disposed on an integrated circuit.
2. Related Art
A die temperature sensor circuit outputs a signal that is proportional to absolute temperature (PTAT) of a die on which the die temperature sensor circuit is disposed. FIG. 1 is a schematic diagram of a known die temperature sensor circuit (hereinafter “known sensor circuit”) 100. The known sensor circuit 100 comprises three MOS operational amplifiers 111, 112 and 113. MOS operational amplifier 113 is the primary amplifier. MOS operational amplifiers 111 and 112 act as buffers. The known sensor circuit 100 comprises bipolar transistor 101 that has its emitter terminal coupled to a current source 134 with an output current equal to NIIBIAS and its base and collector terminals coupled to VSS. In one known sensor circuit 100, NI=4. The known sensor circuit 100 also comprises bipolar transistor 102 that has its emitter terminal coupled to a current source 131 with an output current equal to IBIAS and its base and collector terminals coupled to VSS.
An input signal of the known sensor circuit 100 is a temperature of the known sensor circuit. More specifically, the input signal is a junction temperature of bipolar transistors 101 and 102, which are assumed to have a same temperature. Bipolar transistors 101 and 102 are biased such that their current densities are different and such that a ratio between their current densities remains constant with temperature. A difference between a base-to-emitter voltage of bipolar transistor 101 and a base-to-emitter voltage of bipolar transistor 102, or ΔVBE, is PTAT. A ratio between emitter area of bipolar transistor 102 and emitter area of bipolar transistor 101 is NA. Therefore, the current density of bipolar transistor 101 is NI×NA greater than the current density of bipolar transistor 102. In one known sensor circuit 100, bipolar transistor 101 is one-quarter the size of bipolar transistor 102; therefore, the current density of bipolar transistor 101 is sixteen times greater than the current density of bipolar transistor 102. The term “size” means emitter area.
MOS operational amplifier 113 and resistive elements 121, 122, 123 and 124 correspond to a classical MOS difference amplifier, where resistive elements 121 and 122 have equal values of resistance, and resistive elements 123 and 124 have equal values of resistance. A ratio between resistive elements 123 and 121 is the same as a ratio between resistive elements 124 and 122, and is equal to a gain of the MOS difference amplifier 113. The ΔVBE is inputted to the MOS difference amplifier 113 via MOS operational amplifiers 111 and 112. The ΔVBE is amplified by the MOS difference amplifier 113. However, MOS transistors (not shown) are used to implement the input differential pairs of MOS operational amplifiers 111, 112 and 113, and MOS differential pairs disadvantageously have an input offset voltage that creates an error that is multiplied by a gain of its operational amplifier. Furthermore, MOS operational amplifiers 111 and 112 each has its own input offset voltage error which is disadvantageously multiplied by a gain of MOS difference amplifier 113. The input offset voltages of MOS operational amplifiers 111, 112 and 113 are added to the ΔVBE signal generated by bipolar transistors 101 and 102, and are amplified as an error component. Because the known sensor circuit 100 uses three MOS operational amplifiers, input offset voltages are the biggest contributors of error in the known sensor circuit 100. Each MOS operational amplifier 111, 112 and 113 has an input offset voltage that varies with temperature in a non-linear manner, and, therefore, in manner that is non-PTAT. Therefore, the input offset voltage errors cannot be corrected by a linear method of calibration. With the known sensor circuit 100, three operational amplifiers 111, 112 and 113 are needed, thereby requiring large die area.
The known sensor circuit 100 comprises a first resistive element 121, a second resistive element 122, a third resistive element 123 and a fourth resistive element 124. Typically, each resistive element corresponds physically to one or more unitary resistors (not shown) connected in parallel and/or in series to produce a desired resistance value. The known sensor circuit 100 depends upon each of the unitary resistors to be of equal value, as measured in ohms. Typically, the first and second resistive elements 121 and 122 each comprises one unitary resistor, and the third and fourth resistive elements 123 and 124 each comprises twenty unitary resistors. The gain of the known sensor circuit 100 is set by a ratio of resistance of the first and second resistive elements 121 and 122 to resistance of the third and fourth resistive elements 123 and 124. The unitary resistors must be precisely matched, i.e., the value of the first resistive element 121 must be equal to the value of the second resistive element 122, and the value of the third resistive element 123 must be equal to the value of the fourth resistive element 124. The ratio of resistance of the first resistive element 121 to resistance of the third resistive element 123 must be well defined and must remain constant regardless of temperature. Also, the ratio of resistance of the second resistive element 122 to resistance of the fourth resistive element 124 must be well defined and must remain constant regardless of temperature. With the known sensor circuit 100, any mismatch among the unitary resistors results in temperature coefficient errors and causes sensitivity to the actual value of VBE, i.e., disadvantageously causes common-mode signal amplification.
There are other sources of error, e.g., common-mode rejection ratio and supply rejection, which are highly dependent on temperature. Because these sources of error are highly dependent on temperature, they affect the measurement of the ΔVBE of bipolar transistor 101 and bipolar transistor 102. If the value of current source NIIBIAS 134 is not actually NI times the value of the current source IBIAS 131, where NI is a constant, significant output error and linearity issues result, which cannot be cancelled by a linear method of calibration.
The bipolar transistor base-to-emitter voltage, VBE, is function of the collector current. With the known sensor circuit 100, the bipolar transistors 101 and 102 are biased through the emitter terminals. Therefore, VBE1 and VBE2 disadvantageously depend on the current gain, β, which is weakly dependent on temperature and on biasing currents. As a result, ΔVBE thermal variation is sensitive to β1 and β2 when β1≠β2. Therefore, VOUT is disadvantageously sensitive to the β of bipolar transistor 101 and to the β of bipolar transistor 102. The voltage ΔVBE has a very low thermal coefficient, typically about 0.1 millivolt per kelvin (mV/K), so it needs to be amplified to produce a desired output voltage.
If resistive elements 121 and 122 are not perfectly equal, and/or if resistive elements 123 and 124 are not perfectly equal, not only would the thermal coefficient disadvantageously change, but also MOS amplifier 113 would disadvantageously amplify a common-mode signal that is temperature dependent, i.e., the output thermal coefficient would be adversely affected in a non-linear manner. In one known sensor circuit 100, resistive elements 121 and 122 each have a resistance R and resistive elements 123 and 124 each have a resistance 20R. In such known sensor circuit 100, if R≠R and/or 20R≠20R, not only would the thermal coefficient disadvantageously change, but also MOS amplifier 113 would disadvantageously amplify a common-mode signal. Linear calibration cannot compensate for such a non-linear effect on the thermal coefficient.
In another known sensor circuit (not shown), the value of the first and second resistive elements 121 and 122 is set very high to reduce the current flowing through them, the MOS operational amplifiers 111 and 112 are omitted, and the first and second resistive elements are connected directly to the emitter terminals of bipolar transistors 101 and 102. Although this other known sensor circuit (not shown) eliminates the input offset voltage caused by operational amplifiers 111 and 112, the current flowing though the first and second resistive elements 121 and 122, albeit small, flows through bipolar transistors 101 and 102 and distorts the measurement of the absolute temperature.